Conventionally, Ag and Ag alloys have been used as electric contact materials, which have a high electric conductivity and good resistance to oxidation.
On the other hand, there was a problem that the contact exposed to a high temperature resulting from an arc induced by an electric current being turned on or off causes adhesion of melted contact. For example, the adhesion due to melting may occur in a thermal fuse by generating an arc induced between a movable electrode and a lead wire which are responsible for turning on or off of the current.
In contrast to this, a thermal fuse free from an adhesion trouble due to melting is described in the International Publication No. WO 03/009323. This thermal fuse can be provided with a movable electrode formed of a material that can be obtained by internally oxidizing an alloy composed of 99 to 80 parts by weight of Ag and 1 to 20 parts by weight of Cu so as to make an oxide-lean surface layer thereof in a thickness of 5 μm or less, with the average particle diameter of the oxide particles present inside the alloy being 0.5 to 5 μm.
According to the description, the material used for the movable electrode of the thermal fuse disclosed in the International Publication No. WO 03/009323 allows an oxide-lean layer to exist on its surface layer so long as it is 5 μm or less in thickness. In fact, according to first to 18th examples described in Patent Document 1, the oxide-lean layer of any of the examples is not 0 μm but 1 to 4 μm in thickness, thus allowing the presence of the oxide-lean layer on the surface layer.
However, the present inventor has found through studies that the oxide-lean layer present on the surface layer, even with a thickness thereof being 5 μm or less, can readily cause adhesion due to melting. Thus, it cannot be said that an electric contact formed of the material described in the International Publication No. WO 03/009323 had satisfactorily addressed the problem of adhesion due to melting.